Generally, metal halide discharge lamps available in todays market are of the intermediate or high wattage variety, i.e., 175 to 1500 watts for example. Also, these higher wattage metal halide lamps have the higher efficacy which is directly related to efficiency and conveniently defined as the ratio of total lumen output to input power as expressed in lumens per watt. Moreover, it is known that the efficacy of a lamp tends to decrease as the wattage decreases. Thus, it has been generally presupposed that low wattage metal halide lamps, wattages of 100-watts or less, would be entirely unsatisfactory in so far as efficacy is concerned.
Another common practice in intermediate and relatively high wattage metal halide lamps is the provision of an inert fill gas in the outer envelope surrounding the arc tube. Accordingly, it was believed that the inert fill gas would prevent oxidation of the metal parts located in the outer envelope and increases the breakdown voltage whereby arcing would be inhibited. However, it was found that the undesired loss of heat due to convection currents made possible by the presence of the inert gas significantly reduced the efficacy of the discharge lamp.
Known attempts to reduce this undesired heat loss due to convection currents include the utilization of a glass cylinder surrounding the arc tube within the sealed envelope. However, structures which include a fill gas in the outer envelope do have undesired convection currents. Moreover, these convection currents and the accompanying heat loss are present even though a glass cylinder like arrangement is employed.
Further, the smallest known domestic commercialized metal halide discharge lamp is a Sylvania 175 watt lamp formed for horizontal operation only and having a color temperature of about 3000.degree. K. This lamp has a phosphor-coated outer envelope which transforms UV radiation into visible red radiation thereby creating a relatively warm color appearance. However, the lamp undesirably requires a relatively large and cumbersome luminaire for adequate light distribution control, in addition, has the disadvantage of being operable in a horizontal position only.